Light-emitting diodes are usually used as a light-emitting element in optical communications technology using optical fibers. The emitter-grounded or collector-grounded circuits in shown FIGS. 1(a) and (b) are referred to as the drive circuit for these types of LEDs. By means of these circuits, the light-emitting diode is controlled by voltage and therefore, the circuits do not have a structure with which the light emission is precisely controlled and thus, they are not appropriate for use in high-performance optical communications. That is, in high-performance optical communications control of the light emitted from a light-emitting diode should be precision current-controlled.
On the other hand, application of optical communications in recent years has also led to a demand for increased drive speed. A structure that eliminates unnecessary inductance by common connection of one side, for instance, the cathode side, of the light-emitting diode at a constant potential, such as a ground potential, is preferred in order to satisfy this demand.
An example of a circuit that has been proposed in order to control the emission of light by light-emitting diodes by controlling current with a structure wherein one side of the light-emitting diode has a common connection is shown in FIG. 2. By means of this circuit, light-emitting diode D1 is controlled on or off by differential by one pair of NPN-type transistors Q1 and Q2, the current that flows to the collector side of Q4 is controlled by a current mirror circuit that uses a pair of PNP-type transistors Q3 and Q4, and the light emitted by light-emitting diode D1 is thereby controlled. However, control using a current mirror circuit cannot be applied in high-speed switching, and the use of PNP-type transistors that operate relatively slowly makes high-speed operation of the drive circuit difficult.
Also, FIG. 3 shows an example of a drive structure that has been proposed as a structure that does not have outside control and does not use a PNP-type transistor or current mirror circuit. By means of this example, resistor R1 is connected to the anode side of a light-emitting diode whose cathode is common connected, transistor Q6 is further connected to resistor R1, and this resistor and transistor approximately determine the drive current of light-emitting diode D1. Moreover, transistor Q5 is positioned in parallel with resistor R1 with the base thereof connected between resistor R1 and transistor Q6 and the collector side thereof connected to constant-current source I3 and the base of transistor Q6. By using this type of structure, it is possible to create a pseudo-constant current from the voltage between the base emitters (VBE) of NPN-type transistor Q5 and resistance R1. However, this circuit poses a problem because there are fluctuations in the current that flows through light-emitting diode D1 with the voltage between the base emitters, it is difficult to reliably control the circuit so that a stable current can be supplied.
An example of a circuit that is a modification of the circuit in FIG. 3 is shown in FIG. 4. The intention of this circuit is to very precisely control current by placing differential amplifier U1 wherein the emitter side of transistor Q6 and the output side of constant-current source I3 serve as inputs to this amplifier. However, by means of this type of circuit, a differential amplifier is necessary, which increases circuit size and makes it difficult to guarantee stability of the entire circuit.
Therefore, the present invention provides an LED drive circuit capable of outside control with which high-speed operation of a light-emitting diode at a high frequency is possible and stable light emission by the light-emitting diode can be guaranteed.
Furthermore, the present invention provides a relatively small circuit with a simple structure with which light emission from a light-emitting diode can be precisely controlled and stable operation thereof can be guaranteed.